Phased arrays are systems of multiple spatially diverse (spaced apart) antennas (a/k/a antenna arrays) that can focus signal energy into a narrow beam radiating into/from specific directions and electronically change the direction of signal transmission and reception. These beam forming schemes reduce the interference levels by placing a null in undesired directions, increase the effective SNR, and conserve the battery power by focusing the energy only at desired directions. Phased arrays have been used for radar, imaging, and communications in military, space, medical, and commercial applications due to their ability to form electronically steerable beams. Phased arrays have been limited to discrete implementations where physically separated antennas are connected to their associated electronics (separate electronics for each antenna). This approach leads to an increase in system cost, size, and power consumption.
Phased arrays, also known as electronically steered arrays (ESA), imitate the behavior of directional antennas whose bearing can be adjusted electrically. They use multiple spaced antennas and can shape the transmitted or received electromagnetic beam (beam forming). In phased array receivers, the incident wave reaches spatially apart antenna elements at different times. This time delay difference is a function of antenna spacing and angle of arrival. The receiver compensates for these time delays and combines all the signals to enhance the reception from the desired direction, while rejecting emissions from other directions (spatial selectivity). The coherent addition of signal and the uncorrelated nature of noise in these systems improve the output signal-to-noise ratio by the array size.
Phased array transmitters provide appropriate time delay for the signal at each antenna element so that the radiated electromagnetic beam from the array has the intended shape (e.g., pointed to a specific angle). When a linear array of uniformly spaced antennas receives a plane electromagnetic wave, each antenna receives a successively-time-delayed version of the signal, with the inter-element delay depending on the inter-antenna separation and the angle of incidence of the wave. The signal can then be recovered with maximum power gain by compensating for the inter-element delay electronically in the receiver. Consequently, exercising electronic control over the time-delay in each antenna's signal path allows one to “look” for electronic beams in different directions.
The design of any high performance integrated communication system begins with the transceiver architecture. Phased arrays and transmit/receive spatial diversity systems are the main applications of multiple antenna transceivers. Phased arrays imitate the behavior of directional antennas whose bearing can be adjusted electrically. They compensate the time delay differences of the radiated signal between the antenna elements, and combine the signals to enhance the reception from the desired direction, while rejecting emissions from other directions. The coherent addition of signal and the uncorrelated nature of noise in these systems improve the output SNR by the array size. Providing the delayed version of the transmitted or received signal is a common unique feature of various multiple antenna schemes. In narrowband implementations, this time delay can be approximated with a constant phase shift. This approximation does not hold as the signal bandwidth gets larger and causes signal distortion. Controlling the signal time delay in each path of a phased-array radio can be achieved by various methods involving multiple trade-offs in the performance of phased-array systems. Additionally, the amplitude of each path in multiple antenna systems can be individually controlled in order to increase the SNR and reduce the gain at undesired incident angles (controlling side-lobe levels and null location in the EM beam pattern).
FIG. 1 includes FIGS. 1(a)-1(b), which show two conventional approaches 100a and 100b to phase shifting and power combining in the signal path for a phased array of antenna 106(1)-106(N) coupled to amplifiers 102(1)-102(N). In narrowband systems such as shown in FIG. 1(a(, the most straightforward method of adjusting the signal time delay is by providing a variable phase shifter 104(1)-104(N) at the bandwidth of interest in each signal-path. The loss inevitable with most integrated variable RF phase shifters reduces the receiver sensitivity and the transmitter radiated power. Active implementations of RF phase shifters can eliminate loss at the expense of increased nonlinearity and power consumption. However, by phase shifting and signal combining at RF, more radio blocks are shared resulting in reduced area and power consumption. Additionally, since the unwanted interference signals are cancelled after signal combining, e.g., by combiner 108, the dynamic-range requirements of the following blocks (both linearity and noise figure) such as mixer 110 coupled to local oscillator 112 and A/D 114, are more relaxed. If amplitude control is needed, it can be achieved by variable-gain low-noise amplifiers before or after the phase shifters at RF.
As depicted in FIG. 1(b), phase shifting and signal combining can also be performed after down-converting the received signals to an intermediate-frequency (IF). In FIG. 1(b), amplifiers 152(1)-152(N) are coupled to antennas 156(1)-156(N). A mixer 158(1)-158(N) connected each amplifier 152(1)-152(N) to a phase shifter 154(1)-154(N) after mixing with a local oscillator 160. The phase shifter are connected to a combiner 162, which in turn is coupled to A/D 164.
With continued reference to FIG. 1(b), due to the additional signal amplification at the RF stages, phase shifter loss will have a less deteriorating effect on receiver sensitivity in case it is performed at the IF stage, However, some of the aforementioned advantages, including a lower dynamic-range requirement for the RF mixer, become less effective. Moreover, the value of passive components needed to provide a certain phase shift is inversely proportional to the carrier frequency. Since the value of integrated passive components is directly related to their physical size, passive phase shifters at IF consume a larger area.
In wideband systems, a true variable time delay element should be places in each signal path. For example, elements 104(1)-104(N) in FIG. 2(a) could be replaced with true variable time delay elements. The time delay of an EM wave can be varied by either changing the propagation velocity, altering length of signal propagation, or a combination of them.
FIG. 2 includes FIGS. 2(a)-2(b), which show two alternative conventional approaches 200a and 200b to phase shifting and power combining for a phased array. As shown in FIG. 2(a), phase shifting can be accomplished in the local-oscillator (LO) path. The phase of the received signal can indirectly be varied by adjusting the phase of local-oscillator signal used to down-convert the signal to a lower frequency. This is due to the fact that the output phase of a multiplier (or mixer) is a linear combination of its input phases. FIG. 2(a) shows a simplified phase-array receiver that uses LO phase shifting. Amplifiers 202(1)-202(N), mixers 204(1)-204(N), antennas 206(1)-206(N), and phase shifters 208(1)-208(N) are configured as shown. A local oscillator 210 is used in conjunction with combiner 212 and A/D 214.
Phase shifting at the LO port is advantageous in that the phase shifter loss and nonlinearity does not directly deteriorate the receiver dynamic range or transmitter radiated power. However, since the undesired interferences are only rejected after the combining step at the IF, RF amplifiers and mixers need to have a higher dynamic range compared to the ones in the signal-path phase shifting scheme. The increased number of building blocks might also increase the chip area and power consumption of the receiver. The control of signal amplitude can be made possible more easily with IF variable-gain amplifiers (VGA). It should be reminded that since the frequency of the local oscillator is fixed, the exact path delay can be maintained for only a single RF frequency. In other words, LO phase shifting is not an efficient solution for wide-band RF signals.
FIG. 2(b) depicts a digital array architecture used for a conventional phased array application. The delay and amplitude of the received signal can be adjusted at the baseband using a digital processor, as shown in FIG. 2(b). In FIG. 2(b), LO 250 is applied to mixers 252(1)-252(N) configured between amplifiers 250(1)-250(N) and A/D 256(1)-256(N). Amplifiers 250(1)-250(N) are connected to antennas 254(1)-254(N). DSP 260 is connected to A/D 256(1)-256(N) as shown.
Digital array architectures such as shown in FIG. 2(b) can be very flexible and can be adapted for other multiple antenna systems used for spatial diversity such as multiple-input multiple-output (MIMO) schemes. Despite its potential versatility, baseband phased-array architecture uses a larger number of components compared to the previous two approaches, resulting in a larger area, more power consumption, and higher system complexity and cost. At the same time, since the interference signals are not cancelled before baseband processing, all the circuit blocks, including the power-hungry analog-to-digital converters (ADC), need to have a large dynamic range to accommodate all the incoming signals without distortion. Above all, handling and processing a large amount of data through multiple parallel receivers can be challenging even for today's advanced digital technology. An illustrative example is shown in FIG. 2(b), where a baseband data-rate of 1.92 Gb/s is required. As a comparison, the fastest rate for sending the data into a personal computer using today's PCI standard is 32 bits×33 MHz=1.056 Gb/s. This rate is almost halved when notebook computers are used (e.g., IEEE1394 standard supports 400 Mb/s). Alternatively, a very powerful digital signal processing (DSP) core can be used to process this large influx of data, but it is going to be bulky, power-hungry, and expensive in today's technology.
In short, until faster and more power efficient digital data processing becomes available at a lower price, digital implementations still is not an optimum solution for low-cost low-power multiple-antenna systems in hand-held applications (e.g. personal RADAR) or in sensory networks.
In addition to the aforementioned conventional approaches, coupled oscillator array have been utilized. In such, a linear array of identical second-order oscillators were each oscillator is coupled to its neighboring oscillators has a stable steady state response: all oscillators generate a sinusoid signal with the same frequency and phase. Upon manually controlling the phase of boundary oscillators, all oscillators will still generate the same frequency but with a linear phase progression from one end to the other end. In a linear narrowband phased array, the EM plane wave radiates from adjacent antenna elements with a linear phase difference. Therefore, coupled oscillator arrays are attractive solution to phased array implementations without using explicit phase shifters. The simple nearest neighbor coupling of oscillators intended for a phased array application has serious drawbacks in practical implementations. Due to unavoidable mismatches in any implementation, the free running frequencies of integrated oscillators in an array are unequal and can vary by at least 1%. This seemingly small random frequency variation is sufficient to cause a significant undesired phase shift. The desired phase shift between adjacent coupled oscillators have been set by (i) manually tuning the free running frequency of each oscillator, or (ii) by having adding phase locked loops for all adjacent oscillator pairs. The first approach is not suitable for dynamic adjustment of phases for beam steering while the second approach adds to the system cost, complexity, and power consumption.
Phased arrays find wide use in radar, radio astronomy, remote sensing, electronic warfare, spectrum surveillance, wireless communications, and imaging applications. The advantage of phased array systems is more noticeable as the number of elements in the array is increased. However, in conventional architectures, that translates to a significant increase to the size, cost, and power consumption of the overall system. Phased array architectures that reduce the size and power consumption, while preserving the same performance, are highly desirable.
Higher frequencies offer more bandwidth for ultra high data rate wireless communications and better resolution for radar and imaging systems, while reducing the required size of integrated systems in a multi-antenna configuration. Most of the existing phased array systems are implemented using expensive processes and devices such as compound semiconductors. The reduction of minimum feature size in the metal oxide semiconductor (MOS) transistor, accompanied by other rules of scaling has resulted in a more dense integration and higher operation speed at a lower cost for these circuits. Integration of a complete multi-antenna system in a standard low-cost silicon process technology, especially CMOS, results in substantial improvements in cost, size, and reliability and provides numerous opportunities to perform on-chip signal processing and conditioning. Although newer silicon processing technologies offer transistors capable of operation at higher frequencies at a lower power consumption, other issues such as the reduction of power supply, the low quality of integrated passive components, mismatch between integrated components, and multiple sources of noise and interference propagating in a conductive silicon substrate have hindered the proportionate advancement of analog integrated communication circuits at extremely high frequencies. Hence, system architectures and circuit techniques that allow for fully integrated silicon-based phased array solutions at radio-frequency (RF), microwave, and millimeter-waves are very attractive.
In the recent past, the integration of phased array transceivers using silicon-based technologies has aroused a large amount of interest. These efforts are primarily motivated by economics, as silicon-based technologies are far more cost-effective than more exotic technologies. However, the integration of phased array transmitters poses a number of challenges to the analog designer—a simple replication of the signal path for each antenna results in significant area and power consumption.